This invention relates to a crystal resonator used for measuring pressure in fluids.
It is necessary or desirable to measure pressure in a variety of physical environments. One of the most caustic environments in which pressure measurement is necessary is that of deep oil and gas wells. One of the present techniques used in measuring pressure in such environments involves the use of a quartz crystal transducer apparatus which includes a circular resonator section peripherally supported within a hollow, cylindrical housing formed as an integral part of the resonator section. See. U.S. Pat. Nos. 3,617,780 and 3,561,832. The resonator section of such apparatus is caused to vibrate by oscillatory electrical signals applied to electrodes placed on the resonator section. The frequency of vibration of the resonator section varies with variation in radially directed stresses in the resonator section caused by pressure on the housing. Variation in the frequency of vibration of the resonator section thus affords a measure of the pressure to which the housing is subjected.
One of the problems in using apparatus such as that described above for measuring pressure is so-called thermal shock. This arises because the output frequency of the resonator shifts as a result of temperature changes, but more particularly as a result of rapid temperature change, referred to as temperature transients. When the apparatus is used to measure pressure in oil and gas wells, there can be significant temperature changes as the apparatus is lowered into and brought back up from the well, and since the change in frequency of vibration is used to measure the pressure, temperature-caused errors occur. The temperature transient effects on the resonator section are caused by temperature gradients which produce radially directed stresses. These temperature gradients result from the flow of heat into both the resonator section housing and into the resonator section from the metal electrodes coating the surfaces of the section.
Another problem with the presently used quartz resonator transducers is that the scale factor (frequency versus pressure slope) is also temperature dependent. In order to compensate for the temperature-induced errors, it is necessary to provide a temperature measurement which can be used to correct the frequency output. However, such temperature measurements must be taken some distance away from the location of the quartz resonator transducer, and so accurate temperature measurements at the location of the transducer are difficult.
It has been found that by appropriate orientation of the crystallographic axes of the quartz resonator transducer, in particular SC-cut quartz crystals, the output frequency of the transducer can be made temperature transient independent. In effect, the frequency of the quartz resonator is made independent and immune from uniform radially directed stresses caused by temperature gradients. However, in the process of eliminating the effects of temperature transients, the ability to measure uniform, radially directed stresses caused by pressures on the transducer housing is also eliminated in prior art devices.
In U.S. Pat. No. 3,561,832, it is suggested that slots be selectively located between the periphery of the resonator section and the shell or housing--in effect, that the resonator section be held in place by tabs extending between the section and the housing. By using two axially displaced resonator sections held in place in this fashion, it is felt that the temperature-dependent properties of the sections can be cancelled out. However, using this configuration for measuring high pressures would result in high stress concentration at the tabs and possible cracking. Also, the configuration is more complicated in design and construction and this, in turn, would likely reduce reliability.